Catalytic cracking is the process of breaking larger hydrocarbon molecules into smaller hydrocarbon molecules through contacting the larger hydrocarbon molecules with a catalyst at reaction conditions. The catalytic cracking process is one method used to produce ethylene and propylene from hydrocarbon feedstocks. The ethylene and propylene are important chemicals for the production of the respective plastics polyethylene and polypropylene, two important plastics having a wide variety of uses, such as a material for fabrication of products and as a material for packaging. Other uses of these chemicals include the production of vinyl chloride, ethylene oxide, ethylbenzene and alcohols. Hydrocarbons used as feedstock for light olefin production include natural gas, petroleum liquids, and carbonaceous materials including coal, recycled plastics or any organic material.
Currently, the majority of propylene production is from steam cracking. However, the demand for propylene is growing faster than the ability to increase production of propylene with steam crackers. Fluid catalytic cracking (FCC) provides an alternative method of meeting the demand for the production of propylene.
One process for enhancing propylene yield is disclosed in U.S. Pat. No. 4,980,053, where a deep catalytic cracking process is disclosed. The process requires 5-10 seconds of contact time, and uses a mixture of Y-type zeolite and a pentasil, shape-selective zeolite. However, the process reports relatively high yields of dry gas.
Other patents disclose short catalyst contact times, but do not recognize significant light olefin yields, such as in U.S. Pat. No. 5,965,012 which discloses an FCC process. The process has a catalyst recycle arrangement with a very short contact time of the feed with the catalyst. However, further cracking takes place in a contacting conduit where regenerated and carbonized catalyst contacts the feed, and not in the riser. Another FCC process is disclosed in U.S. Pat. No. 6,010,618 where there is a very short catalyst and feed contact time in the riser, and the cracked product is quickly removed below the outlet of the riser. Other patents, such as U.S. Pat. No. 5,296,131 disclose very short FCC catalyst contact times, but these processes are operated to improve gasoline production rather than production of light olefins.
Other patents, U.S. Pat. Nos. 4,787,967, 4,871,446, and 4,990,314, disclose the use of two component catalysts used in FCC processes. The two component catalyst systems use a large-pore catalyst for cracking large hydrocarbon molecules and a small-pore catalyst for cracking smaller hydrocarbon molecules.
To enhance propylene yields, shape selective additives are used in conjunction with conventional FCC catalysts containing Y-zeolites. The additives all have essentially the same selectivity characteristics. The problem with current catalysts is that selectivity is limited, and the amount of propylene produced is only a function of the amount of additive used in the catalyst mixture. The propylene yield reaches a maximum at a crystalline shape selective zeolite content in the catalyst blend of approximately 10%.
At high levels of additives, the cracking activity is reduced, and this results in lower conversion. To overcome this, the temperature is increased to increase yield, but at the cost of increased coking of the catalyst, and thereby reducing the catalyst life and yields and requiring more frequent regeneration of catalyst, and at an increase in the undesirable yields of dry gas, or methane and ethane.
While much research has gone into trying new catalysts for enhancing propylene production, increasing light olefin production through other means can overcome limitations of catalysts in the cracking process.